LAS VEGAS — Tiffani’s blond hair falls around her face as she leans forward to sign another autograph. She scrawls a quick note on the glossy publicity photo then puts on a sultry smile as the fan comes to stand beside her for a photo.

His grin widens as she grabs his hands and wraps them around her body.

"There you are," she says, as he finally steps away, several poses later. "Thank you, sweetheart."

The line to meet Tiffani at the annual Adult Entertain Expo in Las Vegas is more than a dozen people long — there are college-age young adults, overweight, balding men and even middle-aged couples. All of them hold cameras, some even snapping pictures while they wait, zooming in on Tiffani’s tight red jeans and sheer black shirt.

She'll do this for four days, as thousands of fans flood through the Hard Rock Hotel and Casino in Las Vegas to be part of the annual Adult Video Network Adult Entertainment Expo, the largest pornography industry trade show in the US that draws between 20,000 and 30,000 people each January.

Formerly a back-alley, mafia-funded industry, pornography has exploded into a socially ubiquitous form of entertainment, evidenced by the throngs that roam the convention halls, snapping photos of their friends embracing porn stars to share via Instagram and Facebook.

Though Las Vegas is, by its own definition, a moral outlier, a growing number of experts are concerned with the way the entire country has accepted, and even embraced, pornography's cultural infiltration.

"The real issue is not whether (porn) has become worse," says Michael Kimmel, a sociologist at Stony Brook University in New York who studies gender, sexuality and masculinity. "The thing that's most important is that it's far more pervasive with far less apology."

An estimated 40 million Americans visit a porn site at least once a month, and 25 percent of all search engine requests in the U.S. are for porn. The heaviest use is among young men: in a 2009 survey of 30,000 college students, more than 10 percent said they viewed pornography online from five to 20 hours a week, and 62 percent said they watched Internet pornography at least once a week. Another study by researchers at Brigham Young University in 2007 found that 21 percent of all college students said they watch porn “every day or almost every day.”

Yet despite how "popular" pornography becomes, it cannot remain unexamined, say media scholars and medical professionals, who warn that failing to address the growing tangle of concerns — specifically the way pornography changes the brain — will come with dire consequences. Most children will have seen porn by the time they’re 11, if not younger, and 79 percent of that exposure will happen in the home — often through innocently misspelled words, pop-up windows or misleading websites, according to the report, “Online Victimization of Youth: Five Years Later,” as published by the National Center for Missing & Exploited Children.

Researchers have also found a correlation between early pornography use and early sexual behavior as well as links between the type of pornography consumed and the increased sexual aggression of the viewers. There’s also evidence pornography is damaging relationships: At a meeting in 2003 of the American Academy of Matrimonial lawyers, two-thirds of the attorneys present said that compulsive Internet use played a significant role in divorces that year, and that in 56 percent of those cases one partner had an obsessive interest in online pornography.

"This is a public health crisis — the fact that porn is now the major form of sex education in the western world," says Gail Dines, a professor of sociology and women’s studies at Wheelock College in Boston, and author of "Pornland: How Porn Has Hijacked Our Sexuality.” "The fashion industry shapes the way we dress, the food industry shapes the way we eat, how would it be possible that the sex industry is the only industry that didn't shape human behavior? How it shapes it is complicated but you cannot walk away from those images unchanged. That's not how we operate."

Addiction chemistry

More than 1,200 miles away from Las Vegas, 25-year-old Gabe Deem sits in the back of the YMCA bus as it rumbles through a suburb of Dallas.

He's surrounded by high school students on their way to the local Y to get help on homework and hang out in a safe place until their parents can pick them up.

Most of the kids have their cellphones in hand, texting and tweeting as they talk.

During a lull, Deem glances across the aisle where a 12-year-old boy and his friend are scrolling through Instragram pictures on their iPhones. All of a sudden, there's a picture of a stripper.

"You gotta get rid of this," Deem tells the kid as he snatches the phone and quickly scrolls past the image. He tries to explain how looking at stuff like that will mess you up, but the boy shrugs it off. He's seen worse.

"There's nothing more frustrating when you're trying to give something up than having the whole society encourage it and say it's not a big deal," Deem says.

Deem’s curiosity with pornography began at age 8 when he found a magazine in the woods near his house. His interest grew when his family bought cable two years later. By the time he was 12 and they got high-speed Internet, he was hooked.

"My parents didn't put any blocks on," Deem says. "They had no clue I'd be doing that, and I was good at hiding it. I saw everything there was to see by the time I was out of middle school."

While there’s no scientific consensus on how porn affects the brain, there is a growing body of research on the subject. One of the most prominent voices on the topic is Gary Wilson, a former science teacher who started a website called YourBrainOnPorn. Wilson believes that viewing and masturbating to pornography can become addictive because that act produces dopamine, the brain’s natural reward for engaging in survival behaviors like mating, eating or conquering.

In animals, a male rat will mate two or three times with a female rat before his dopamine receptors are full and his sex drive is exhausted. However, scientists note that if the male rat meets a new female partner every few minutes, he'll try to mate until he nearly dies of exhaustion — the "Coolidge effect."

Pornography, Wilson believes, has a similar effect on the brain, tricking it into thinking sex is possible with an unlimited number of mates, releasing continual bursts of dopamine and causing a buildup of a learning-related protein in the brain called DeltaFosB. Animal studies show that when subjects engage in overconsumption, whether it's drugs, food or sex, DeltaFosB increases in cells in the brain's pathways, altering the brain's reward system, increasing incentive for the reward and serving as an indicator that addictive behavior is taking place.

“Sexuality is the most powerful natural reward our brain has, in terms of producing a dopamine spike,” says Donald Hilton, a neurosurgeon in Texas who has studied the effects of pornography use on the brain. “Critics who deny the existence of sexual addiction simply don’t understand the brain.”

Wilson started YourBrainOnPorn after he and his wife, who are not religious, began noticing a growing number of visitors to her healthy sexuality website identifying themselves as porn addicts with erectile dysfunction, delayed ejaculation and loss of libido.

Wilson says these symptoms are cropping up in younger men because unlike static Playboy centerfolds or a single DVD, the Internet offers an unlimited number of novel "partners." And with continual dopamine rushes, the brain's receptors become so overworked they shut down, sending the user searching for harder images to feel any pleasure.

"Addictions are chasing after dopamine," Wilson says. "Addictions are wanting more, but liking it less."

Dr. Kent Berridge, a professor of psychology and neuroscience at the University of Michigan, says that advances in neuroscience are leading to a better understanding of how dopamine affects the brain, and how it may lead to addictive behaviors. Three decades ago, he says, researchers thought the release of dopamine caused pleasure.

Yet, over time, they found it's possible to strip away dopamine and still have pleasure, and that boosting dopamine doesn't always mean increased pleasure, though it may increase behavior toward the pleasurable activity.

A study of patients with Parkinson's disease found that those individuals taking dopamine-producing drugs to help with depression reported increased sexual desires.

"This is first evidence that it could be an addiction-like thing for sex, but these are people being given dopamine-stimulated drugs," Berridge says. "Whether that happens in spontaneous sex addicts, that would be an open question. I think it's possible, I just think we don't know yet for sure."

Deem says he initially dropped out of college because he couldn't concentrate on anything besides porn and video games. Relationships weren't working and his sex drive was gone. Deem eventually decided to quit looking at pornography to see if he could “get back to normal.”

Wilson calls this a "reboot," a period of complete abstention from pornography, masturbation and sex to allow the body and brain to rest and recover.

Younger men are taking longer to "reboot" than older men, Wilson says, because older men didn't have their initial brain imprinting and education from high-speed Internet pornography.

Debating definitions

Not everyone buys the idea of a pornography addiction or its consequences, mostly because it's not in the DSM — the Diagnostic and Statistical Manual of Mental Disorders — published by the American Psychiatric Association.

Health care professionals use the DSM to diagnose mental disorders, which are "described strictly in terms of patterns of symptoms that tend to cluster together," according to the APA website.

And that's where the APA differs from the American Society of Addiction Medicine, says Dr. Raju Hajela, director, Region IX (International) of the ASAM, and chair of the subsection of definitions.

"In our definition, behaviors are a consequence of the disease, they are neither the disease nor the cause of the disease," Hajela says. "In the DSM, the behavior is the disorder."

In the newest DSM-5 manual, released in May after a 14-year review, a new category on behavioral addictions includes one entry: "gambling disorder."

Dr. David Kupfer, chair of the DSM-5Task Force, says an Internet Gaming Disorder was considered, but a work group determined "it warranted more clinical research and experience" before it could be officially included and was put in a "future study" section. Pornography addiction wasn't mentioned.

The nod to behavioral addictions is encouraging, but the DSM failed to address the huge problem of pornography, says Hilton, the Texas neurosurgeon who writes about addiction.

"Addiction is what occurs when a reward is pursued despite adverse consequences," he says.

Hilton compares two men, one who sits at a blackjack table for hours waiting for a financial pay-off with a man who stares at a computer all day waiting for an orgasmic pay off. They're both pursuing a reward despite negative consequences but under the new DSM-5, only the gambler would have an addiction.

While Berridge at the University of Michigan hasn't specifically studied pornography, he's interested in the discussion.

"I think it's very plausible that for some individuals each of these things (drugs, porn, gambling) is addictive," Berridge says. "But the question will be, in how many individuals? It's almost certain that the labels will be over-applied to people who don't quite fit that, and that's the danger we run with any label."

Deem says he doesn’t care how he's labeled, nor does he mind if people don't believe his story.

He's too busy mentoring his YMCA kids and arranging discussions at local schools to share how using pornography affected him.

And after 780 days of no porn and no masturbation, Deem says his mental clarity and powers of concentration are sharper than they've ever been and that his energy levels are sky high. He also says he’s happier than he thought possible.

"I know the pain of recovery and finding out that I basically screwed myself up by watching porn my whole life," Deem says. "If (someone) had told me (about the biological consequences), yes, that would have definitely been a help to me, so that's what I'm trying to do."